1. Field of the Invention
This invention relates to a fuel injection system for an internal combustion engine.
2. Description of the Prior Art
In Japanese Unexamined Patent Publication No. 56(1981)-148636, there is disclosed a fuel injection system in which so-called stratified charge is used in a partial load operation region in which high engine output is not required. Mere specifically, in the partial load operation region, fuel is injected in the latter half of the intake stroke so that the fuel and air are not mixed uniformly in the cylinder but are in layers, the air-fuel mixture in the upper portion of the cylinder (where a spark plug is positioned) being rich and that in the lower portion of the cylinder being lean. Though the air-fuel ratio of the mixture surrounding the spark plug must be sufficient to be ignited by the spark plug, the lower layer may be solely of air or of very lean mixture. Therefore, the air-fuel ratio of the overall mixture may be very lean, whereby the specific fuel consumption can be substantially reduced and at the same time, unburned components such as CO and NOx can be reduced. Further, since rich air-fuel mixture is confined in a narrow space around the spark plug and the end-gas zone is occupied by air or a very lean air-fuel mixture, abnormal combustion of the air-fuel mixture is prevented, thereby preventing detonation.
In such a fuel injection system in which stratified charge of stratified combustion is accomplished, it is generally preferred that the termination of fuel injection coincide with the end of the intake stroke. However, since the opening degree of the intake valve is small and the effective area of the intake passage is very limited in the proximity of full closure of the intake valve, and since the fuel injection valve is spaced from the combustion chamber by a distance and accordingly there is a time delay before arrival of the fuel at the combustion chamber after injection from the fuel injection valve, actually, fuel injection should be terminated before full closure of the intake valve.
In the fuel injection system disclosed in the Japanese Unexamined Patent Publication described above, the fuel injection termination timing is fixed at a certain crankshaft angle before the intake valve is fully closed, and the fuel injection starting crankshaft angle at which injection of fuel by an amount depending on the operating condition of the engine is started is calculated according to the fuel injection termination. In the fuel injection system of the prior art in which the fuel injection termination timing is fixed, there is a problem that the stratification of fuel and air in the combustion chamber may distrupted under certain operating conditions of the engine. It has been found that the stratification of fuel and air in the combustion chamber is disrupted by the following causes.
(1) As is well known, blow-back of intake gas occurs in the late period of the intake stroke during heavy-load, low-speed operation of the engine. The blow-back of the intake gas occurs when the pressure in the combustion chamber becomes higher than the pressure in the intake port as the piston moves upward, and therefore can occur when the engine operates at a low speed under partial load.
Considering the relation between the time the blow-back of intake gas occurs and the operating condition of the engine, the time the blow-back occurs generally corresponds to the time the pressure in the intake port upstream of the intake valve balances with the pressure in the combustion chamber, but since the delay in intake due to inertia in intake gas is smaller during low-speed operation of the engine than during high-speed operation of the engine, the time the blow-back occurs becomes earlier as the engine speed is lowered. Further, the time the blow-back occurs can vary with load on the engine and is generally apt to become earlier as load on the engine is reduced.
If the fuel injection termination timing is fixed at a certain crankshaft angle as in the fuel injection system disclosed in the Japanese Unexamined Patent Publication identified above, a part of injected fuel is blown back into the intake port, as the blow-back of intake gas occurs, depending on the operating condition of the engine, to stay therein and to be introduced into the combustion chamber from the beginning of the next intake stroke, thereby adversely affecting the stratification in the combustion chamber.
(2) The time necessary for injected fuel to reach the combustion chamber generally depends upon the injecting speed of the fuel and the distance between the fuel in jection valve and the intake valve. When the timing of arrival of fuel at the combustion chamber is seen through the crankshaft angle, the timing varies with the engine speed. In the fuel injection system described above in which fuel injection is set to be terminated at a certain fixed crankshaft angle, a part of the injected fuel cannot reach the combustion chamber within the intake stroke and is confined in the intake port. The part of the injected fuel confined in the intake port adversely affects the stratification in the combustion chamber in the next intake stroke as in the case of the blowback described above.